We postulate that high dose oral clopidogrel will produce immediate potent antithrombotic protection in patients with coronary artery disease (CAD) undergoing coronary artery catherization. In a single-dose, dose-escalation, timed-response study we propose to establish the dose of clopiogrel and timing after oral administration that immediately inactivates platelet ADP-receptors ex vivo. Each dose will be evaluated in cohorts of aspirin-treated CAD patients undergoing coronary artery catherization. The effects of sequential step-wise escalating doses of oral clopidogrel on ex vivo platelet aggregation induced by ADP, collagen and thrombin receptor agonist peptide (TRAP) and in vivo bleeding time will be assessed one day prior to the catherization procedure.